There are various applications, which require a precise measurement of liquid or gas flow-rates (liquids and gases being hereinafter referred to collectively as “a fluid”. Such precise measurements are particularly required in medical, process monitoring and control applications. Historically, a number of approaches have been developed, involving mechanical, electromagnetic and thermal techniques.
For example, U.S. Pat. No. 6,443,003 discloses a sensor for measuring changes in mass air flow by using one or more thermoelectric devices, each of which serves both as a heating element and as a differential temperature sensor. The thermoelectric device or devices are sandwiched between two surface plates. The sensor operates the device or devices in constant current or in pulsed current mode. The operation in constant current mode involves passing the current through one thermoelectric device to create a temperature differential between the two surfaces. A second thermoelectric device generates a voltage in response to the differential, the voltage being proportional to the air flow-rate.
U.S. Pat. No. 5,237,866 discloses an apparatus for measuring the flow of fluid comprising a tube through which the fluid to be measured may flow, including means for varying the temperature at a selected location on the tube. A plurality of temperature sensors are used to measure the temperature at upstream locations, and at least one sensor is used for measuring the temperature downstream of the selected location. A plurality of separate sensors are used on the input end to provide information to determine the temperature gradient along the tube.
U.S. Pat. No. 4,947,889 describes an apparatus in which a part of a tube, through which a fluid passes, is cooled by means of an electronic cooling element and a flow rate of the fluid is measured based on a temperature of a surface of the cooled tube.
U.S. Pat. No. 4,753,111 discloses a fluid flow meter using a pair of temperature-variable resistance elements as detectors. Both elements are exposed to the fluid flow and supplied with a series of current pulses. The pulses supplied to one element, the reference element, are of very short duration so, that there is no appreciable heating effect; the pulses supplied to the other element are of longer duration and are controlled so as to maintain this element at a fixed temperature differential above the other element, which remains at fluid temperature. The pulse width of the longer duration pulses is a measure of the flow rate of the fluid.
In spite of all efforts, the art has so far failed to provide a fluid flow-meter of simple configuration, which can be used to measure very low fluid flow-rates (also referred to hereinafter as “fluid microflow”) and which is relatively inexpensive.
It is an object of the present invention to provide such a fluid flow-meter for measuring very low fluid flow rates (referred to hereinafter also as “microflowmeter”) suitable for measuring, for instance, the velocity of fluid microflow within a narrow pipe, such as a capillary conduit, a vessel, etc.
It is another object of the present invention to provide a microflowmeter, which can also be used to determine whether components of a fluid, flowing within a narrow conduit, such as a capillary conduit, remain constant.
It is still another object of the present invention to provide a precise method and system for measuring fluid microflow within the narrow tube.
It is a further object of the present invention to provide a method and system, which is relatively inexpensive.
Other objects and advantages of the invention will become apparent as the description proceeds.
As used herein, the term “microflow” indicates the flow of a fluid having a flow rate of the order of magnitude of milliliters per hour or less. However, it should be appreciated that the ability to measure very small flaw rates does not deter in any way from the ability to measure greater flow-rates, and the measurement of such greater flow rates is within the scope of the invention.